Process for making a composite aluminum article

ABSTRACT

A process for making a composite aluminum article includes: treating an anodized, sealed aluminum substrate with a low friction material adherent to said substrate. The resulting substrate is then treated with an aqueous solution or suspension containing at least one salt having an anion, a cation, or both of a divalent metal or a trivalent metal. The process is particularly useful for producing hard anodized cookware, which exhibits improved acid and stain resistance.

CROSS-REFERENCE TO RELATED PATENT APPLICATIONS

This application is claiming the benefit under 35 USC 119(e) of U.S. Application 60/524,907, filed Nov. 26, 2003, incorporated herein by reference in its entirety.

FIELD OF THE INVENTION

The present invention relates to a process for the preparation of a stain-resistant composite aluminum article, such as cookware, and to stain-resistant cookware.

BACKGROUND OF THE INVENTION

Aluminum articles having treated oxidized surfaces are commonly used due to the low friction and high corrosion resistance of their surfaces. Anodized aluminum articles have a thin layer of porous, irregular, coarsely crystalline aluminum oxide formed on their surfaces. Additionally, it is known to apply a thin coating to seal the porous oxide. The coating adheres very strongly and tenaciously to the aluminum substrate and thus making the substrate highly abrasion resistant. Processes for anodizing and sealing aluminum include U.S. Pat. Nos. 4,861,440 and 4,784,732, the respective disclosures of which are hereby incorporated by reference in their entirety.

Coated anodized aluminum surfaces may be used in a variety of applications. In particular, they enjoy wide use in the production of cookware due to their strength, hardness and corrosion resistance. Notwithstanding these benefits, anodized cookware may be susceptible to sticking or staining. Further, acidic food may react with the surface of anodized aluminum cookware and may corrode the hard anodized surface.

Attempts have been made to improve food release, improve stain and acid resistance, and provide easier cleaning of anodized aluminum surfaces. Such attempts include: (i) applying polymer layers of varying thicknesses to the surface; (ii) implementing a longer seal time and thereafter applying a polymer layer; (iii) AC anodizing and (iv) chemical grafting. While application of a mixture of polymers has afforded certain benefits, improved stain resistance, enhanced strength, enhanced hardness and corrosion resistance are still desired.

SUMMARY OF THE INVENTION

In accordance with one aspect, the invention relates to a process for making a composite aluminum article comprising:

-   -   (a) treating an anodized, sealed aluminum substrate with a low         friction material adherent to the substrate thereby producing a         low friction material-treated substrate; and     -   (b) treating the low friction material-treated substrate with an         aqueous solution or suspension containing at least one salt         having an anion, a cation, or both of a divalent metal or a         trivalent metal; thereby producing a composite aluminum article.

In accordance with another aspect, the invention relates to a process for making a composite aluminum article comprising:

-   -   (a) sealing an anodized aluminum substrate;     -   (b) treating the sealed, anodized aluminum substrate with a low         friction material adherent to the sealed, anodized aluminum         substrate, thereby producing a low friction material-treated         substrate; and     -   (c) treating the low friction material-treated substrate with an         aqueous solution or suspension containing at least one salt         having an anion, a cation, or both of a divalent metal or a         trivalent metal;     -   thereby producing a composite aluminum article.

According to one aspect of the invention, sealing is performed with an aqueous solution or suspension containing at least one salt having an anion, a cation, or both of a divalent metal or a trivalent metal thereby producing a sealed, anodized aluminum substrate, wherein the salt solution or suspension substantially saturates the low friction material-treated substrate. The salt used may be a nickel salt and/or a cobalt salt. Preferably, a nickel salt is used. More preferably, the nickel salt is nickel acetate. Alternatively, sealing may be accomplished by immersion in near-boiling water (e.g., water at 200° F.-210° F.).

Preferably, the low friction material that may be used is a blend of food grade polymers. Other types of low-friction materials may also be used.

In a preferred embodiment, the treatment of the low friction material-treated substrate with an aqueous solution or suspension containing at least one salt having an anion, a cation, or both of a divalent metal or a trivalent metal is performed at a temperature of 170-180° F. Preferably, the pH of the solution is between 5.7 and 6.0 and the treatment is conducted for 6 to 15 minutes.

In accordance with another aspect of the invention, cookware is made according to the above-described process. Various types of cookware contemplated include, by way of example, kettles, dutch ovens, pots, pans, skillets, griddles and waffle irons. Cookware according to the invention exhibit improved durability, corrosion resistance and stain resistance.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following describes certain preferred embodiments of the invention. It should be understood that this description is intended merely to be exemplary of the invention.

In a preferred embodiment, a composite aluminum article is prepared in four steps from an aluminum substrate. The aluminum substrate can be pure aluminum or an aluminum alloy and can be wrought, cast or forged. The four steps may be summarized as follows: (i) anodizing; (ii) sealing; (iii) treating with a “low friction material,” preferably a blend of food-grade polymers; and (iv) a second sealing with a solution containing the salt of a di- or tri-valent metal.

The first three of these steps have been found to result in anodized surfaces with good strength and corrosion resistance. However, it has been found that the added sealing treatment with the salt of a di- or tri-valent metal significantly and unexpectedly improved the stain resistance of the resulting composite aluminum substrate. This process delivers better performance than achieved with a single sealing step of the anodized aluminum substrate, even where the single sealing step took place over a longer total duration. Conventional wisdom would have predicted that treating the low friction material-treated substrate with a solution containing the salt of a di- or tri-valent metal would not have any impact on the stain-resistance of the resulting composite aluminum substrate.

In the first step of the process for producing a composite aluminum article, an aluminum oxide layer is formed electrolytically on the surface of the aluminum substrate. Anodization methods well known to the skilled artisan (including, for example, those disclosed in detail in U.S. Pat. Nos. 4,861,440 and 4,784,732) may be used. This step produces an aluminum oxide surface that is integral with the base aluminum and is irregular, coarsely crystalline and highly porous.

As known in the art, the aluminum oxide layer can be “grown” on the aluminum substrate by electrolytic treatment while the aluminum substrate is immersed in an oxidizing acid bath. In a conventional anodizing process, the substrate serves as the anode, and high voltages and current densities are used to form a highly porous aluminum oxide layer. In such a process, an aluminum substrate is immersed in a non-etching acid bath for electrolytically growing aluminum oxide crystals. By way of example, aqueous sulfuric acid comprising about 120-310 g/L concentrated sulfuric acid may be used. Other components known in the art may also be used in the bath.

As known in the art, it is preferable that the acid bath used for anodizing be highly agitated. Optionally, this is performed by air agitation or mechanical means well known in the art. If desired, a conventional wetting agent, such as an alkylaryl polyether alcohol, can also be added to the bath wetting agent. The bath is preferably maintained at a temperature of about 25°-80° F., particularly about 26°-36° F., and the temperature of the bath is not allowed to rise substantially during the electrolytic formation of the oxide layer, particularly when high current densities are used.

In order to obtain the appropriate cellular structure of the aluminum oxide layer, the voltage between anode and cathode is increased gradually from the start to the finish of the process. Different voltage ramp schedules based on current density may be used in accordance with techniques known in the art and will vary depending on the application. (See, e.g., Tool and Manufacturing Engineers Handbook, 4^(th) Edition, Volume 3, which indicates that current densities should be maintained at 20-50 amps per square foot). Preferably, a current density of 20-25 amps per square foot is used. The voltage schedule is preferably increased from 20 volts to 100 volts over the cycle time of the anodizing process, preferably 25 volts up to 80 volts at equal time increment. Particular cycle times may vary depending on the specific article to be anodized desired characteristics of the process.

In a preferred embodiment, the aluminum oxide crystal structure thus formed has a thickness of up to 2 mils, preferably about 1.5 mils. The anodization process may take from about 15 minutes to about 60 minutes, depending upon the desired thickness of the aluminum oxide crystal structure and the aluminum alloy composition, as well as the current density, as understood in the art. Of course, it should be understood that these process parameters are merely illustrative, and can be varied to suit particular applications.

In a preferred embodiment, the resulting anodized aluminum substrate is then rinsed thoroughly with deionized, preferably distilled water, to remove any residue on its surfaces from the acid bath.

In the second step according to a preferred embodiment, the anodized aluminum substrate produced in the anodization step is then “sealed” or “pre-sealed.” Sealing may be performed, for example, by treating the anodized aluminum substrate with an aqueous solution or suspension containing one or more salts with an anion, a cation or both of a divalent or trivalent metal, wherein each of the salt(s) is absorbed into the crystal lattice of the aluminum oxide. Other sealing processes may be used instead of such treatment. For example, the anodized aluminum substrate may be sealed with near boiling water.

In the case of sealing with an aqueous solution or suspension containing one or more salts with an anion, a cation or both of a divalent or trivalent metal, the anodized aluminum substrate preferably is dipped into a bath containing the solution, or a suspension containing the salt or salts, for about 20 minutes. Non-limiting examples of salts that can be used to seal the anodized aluminum substrate are the lower alkanoates (e.g., acetate or formate) of nickel, cobalt, lead, zinc and copper and the ammonium, alkali metal and alkaline earth metal dichromates and molybdates, as well as other conventional salts used for sealing aluminum surfaces. In a preferred embodiment, a nickel salt, such as nickel acetate, is used to seal the anodized aluminum substrate.

It will be appreciated that the anodized aluminum substrate may be sealed more than once. Further, (a) mixtures of salts may be used in each sealing step, and (b) each sealing step may employ a different salt or mixture of salts than the previous sealing step.

Alternatively, as known in the art, it is possible to seal by immersing the anodized aluminum substrate in water that is near boiling (e.g., 200-210° F.) for a sufficient period of time, preferably, about 5 minutes.

In the third step according to a preferred embodiment, the anodized, sealed aluminum substrate is treated with a lubricant or low friction material, thereby producing a low friction material-treated substrate.

Examples of lubricants/low friction materials include, but are not limited to, graphite, silicone, molybdenum disulfide, polymers and the like. In a preferred embodiment, the anodized, sealed aluminum substrate is treated with a blend of food grade polymers by dipping the anodized, sealed aluminum substrate in a bath containing the mixture of polymers for about 5 minutes. One blend of food grade polymers that is preferred is an HCR-F® blend provided from General Magnaplate, Linden, N.J.

The resulting low friction material-treated substrate is then rinsed thoroughly with deionized water, preferably hot, distilled water, before performing the fourth step of the process of a preferred embodiment of the present invention. The use of hot water (e.g., around 180 degrees F.) heats the substrate to facilitate more rapid evaporation of the water, but such heating may not be necessary. After the water rinse, the low friction material-treated substrate is baked so as to remove any residual water that may be found in the lubricant/low friction material. Preferably, baking takes place at a temperature from about 500° F. to about 700° F., most preferably at about 680° F. Baking takes place for an appropriate time to cure, typically at least 6 to 90 minutes, depending, for example, on temperature.

When applied to the production of anodized aluminum cookware, the above described three steps have been found to provide improved food release, acid resistance and easier cleaning. Tests, however, have shown that stain resistance of the resulting product were unacceptable.

In a fourth, and final step, according to a preferred embodiment, the resulting low friction material-treated substrate is again sealed or “post-sealed” with an aqueous solution or suspension containing one or more salts with an anion, a cation or both of a divalent or trivalent metal, as described above. Preferably, the low friction material-treated substrate is dipped in a nickel acetate solution for an immersion time of about 6-15 minutes, at a temperature of 170° F.-180° F., wherein the pH of the nickel acetate solution is between 5.7 and 6.0. This final step produces a composite aluminum article with improved characteristics. In particular, it has been found that this final step can be used advantageously to produce hard anodized cookware with markedly improved stain resistance.

While the invention is in no way limited by any underlying theory for such improved results, it is believed that the composite aluminum article comprises two protective layers when the low friction material-treated substrate is subsequently treated with the above-mentioned salt solution: a first protective layer that is formed by the low friction material; and a second protective layer that is formed when the salt(s) is absorbed into the pores of the low friction material (particularly when the low friction material is a polymer or blend of polymers). While not wishing to be bound by theory, it is believed that these two protective layers, in combination, provide the composite aluminum article with the observed enhanced stain resistance. The resulting article further exhibits superior strength, hardness and corrosion resistance.

The surface of the composite aluminum article produced by the process described herein is ideal for the production of pieces of cookware (e.g., kettles, dutch ovens, pots, pans, skillets, griddles, waffle irons, and the like), where stain-resistance, as well as enhanced strength, enhanced hardness and corrosion resistance are important. The surface of the composite aluminum article exhibits excellent stain-resistance. In addition, the surface of the composite aluminum article exhibits excellent acid resistance and easier cleaning. The fourth step in the process for the production of the composite aluminum article of the preferred embodiments of the present invention significantly and unexpectedly improved the stain resistance of the resulting composite aluminum substrate to blueberry paste, curry paste, cooking oil and tomato.

The composite aluminum article of a preferred embodiment may be produced using the four steps described above (i.e., anodization, pre-seal, lubricant/low friction material treatment, post-seal), starting with an aluminum substrate that is not anodized. It will be appreciated, however, that the composite aluminum article of a preferred embodiment may be produced starting with an anodized aluminum substrate that may be obtained, for example, from a third-party source. The anodized aluminum substrate would then be pre-sealed using the methods disclosed (or methods well known in the art), treated with a lubricant or low friction material and post-sealed according to the methods disclosed above.

Alternatively, a process according to another embodiment of the invention may involve providing a sealed, anodized aluminum substrate and sealing the sealed, anodized aluminum substrate according to the methods disclosed above.

While alternative methods for achieving a stain resistant aluminum article have been explored, many of them have not successfully yielded an article that resists staining as well as the composite aluminum article of the preferred embodiment. Alternative methods that were attempted include the following:

-   -   (a) applying a polymer to an unsealed anodized substrate and         then sealing;     -   (b) chemically grafting a fluoropolymer onto an anodized         aluminum substrate;     -   (c) dipping a sealed, anodized aluminum substrate in a polymer         bath for longer times so as to create a thicker polymer coat,         rinsing the resulting low friction material-treated substrate         with cooler water for a shorter period of time (e.g., room         temperature and 5-10 second duration), without subsequent         sealing of the resulting low friction material-treated         substrate;     -   (d) AC anodizing the initial aluminum substrate.

These alternative techniques were not found to provide the advantages or features provided with the invention.

It is believed that this invention and many of its attendant advantages will be understood from the foregoing description, and it will be apparent that various changes and modifications can be made in the composite aluminum article of this invention and in the process for making the article without departure from the spirit and scope of the invention or sacrificing all of its material advantages, the article and process hereinbefore described being merely preferred embodiments. 

1. A process for making a composite aluminum article comprising: (a) treating an anodized, sealed aluminum substrate with a low friction material adherent to said substrate thereby producing a low friction material-treated substrate; and (b) treating said low friction material-treated substrate with an aqueous solution or suspension containing at least one salt having an anion, a cation, or both of a divalent metal or a trivalent metal; thereby producing a composite aluminum article.
 2. The process of claim 1, wherein said salt solution or suspension substantially saturates said low friction material-treated substrate.
 3. The process of claim 1, wherein said salt is one or both of a nickel salt and a cobalt salt.
 4. The process of claim 3, wherein said salt is a nickel salt.
 5. The process of claim 4, wherein said nickel salt is nickel acetate.
 6. The process of claim 1, wherein said low friction material is a polymer material.
 7. The process of claim 1, wherein the treatment of said low friction material-treated substrate with an aqueous solution or suspension containing at least one salt having an anion, a cation, or both of a divalent metal or a trivalent metal is performed at a temperature of 170° F.-180° F.
 8. The process of claim 7, wherein the pH of said solution is between 5.7 and 6.0.
 9. The process of claim 8, wherein said treatment is conducted for 6 to 15 minutes.
 10. An article made using the process of clam
 1. 11. The article of claim 10, wherein said article is a piece of cookware.
 12. The piece of cookware of claim 11, wherein said piece of cookware is selected from the group consisting of kettles, dutch ovens, pots, pans, skillets, griddles and waffle irons.
 13. A process for making a composite aluminum article comprising: (a) sealing an anodized aluminum substrate; (b) treating said sealed, anodized aluminum substrate with a low friction material adherent to said sealed, anodized aluminum substrate, thereby producing a low friction material-treated substrate; and (c) treating said low friction material-treated substrate with an aqueous solution or suspension containing at least one salt having an anion, a cation, or both of a divalent metal or a trivalent metal; thereby producing a composite aluminum article.
 14. The process of claim 13, wherein sealing comprises immersion in an aqueous solution or suspension containing at least one salt having an anion, a cation, or both of a divalent metal or a trivalent metal, thereby producing a sealed, anodized aluminum substrate.
 15. The process of claim 13, wherein sealing comprises immersion in water heated to near boiling.
 16. The process of claim 13, wherein said salt solution or suspension substantially saturates said low friction material-treated substrate.
 17. The process of claim 13, wherein said salt is one or both a nickel salt and a cobalt salt.
 18. The process of claim 17, wherein said salt is a nickel salt.
 19. The process of claim 18, wherein said nickel salt is nickel acetate.
 20. The process of claim 13, wherein said low friction material is a polymer material.
 21. The process of claim 13, wherein the treatment of said low friction material-treated substrate with an aqueous solution or suspension containing at least one salt having an anion, a cation, or both of a divalent metal or a trivalent metal is performed at a temperature of 170° F.-180° F.
 22. The process of claim 21, wherein the pH of said solution is between 5.7 and 6.0.
 23. The process of claim 22, wherein said treatment is conducted for 6 to 15 minutes.
 24. An article made using the process of clam
 13. 25. The article of claim 24, wherein said article is a piece of cookware.
 26. The piece of cookware of claim 25, wherein said piece of cookware is selected from the group consisting of kettles, dutch ovens, pots, pans, skillets, griddles and waffle irons. 